Hydrostatic altimeter measures elevation directly

DN Staff

March 4, 1996

4 Min Read
Hydrostatic altimeter measures elevation directly

Valley Center, CA--More than most engineers, Dennis Vories likes to keep things on the level. Several years ago, presented with the difficulty of single-handedly measuring elevations for the construction of his office, he decided there must be a better method than the traditional sight level and rod, or laser beacon. So he invented Compulevel, the only device in the world that gives direct elevation readings to within 1/8 inch over a 200-ft circle and a 40-ft vertical range in a single setup.

"This system has three times the vertical capability of conventional equipment in a single setup," he explains. By leap-frogging the two-piece system from one end of its cord to the other, it can cover, theoretically, a n100,000-ft vertical range over an unlimited area, without requiring any tabulation or manual calculations.

Compulevel works on the principle of differential hydrostatic pressure. It consists of two primary components--a base unit (BU) and a measurement module (MM)--joined by a 100-ft-long, dual-bore polymer cord. One bore contains a proprietary hydraulic fluid, while the second holds a gas used to pressurize the system.

The fluids act on a single pressure sensor contained in the MM. Quite simplistically, positive pressures indicate negative elevations (with the MM lower than the BU) while negative pressures indicate positive elevations.

This design pushes the limits of the pressure sensor. "They weren't intended to be used this way," Vories explains. "The sensors are very nonlinear in the negative region."

To avoid this situation, he might have specified two sensors--one in the base and one in the MM. Both would have measured only positive pressures. Such a solution, however, would have driven up costs and increased manufacturing complexity by necessitating a run of fragile wiring between the two units. Instead, Vories devised a way to measure the linearity of each unit's single sensor during the assembly process, and save correction parameters to an onboard EEPROM.

The result: One part in 12,000 resolution (0.050-inch in 40 ft) over a temperature range of 22F to +158F. In fact, with the correction, Compulevel is now imperceptibly more accurate in the negative region than the positive.

All the electronics, including the processor, memory, display, PC data port, and temperature and pressure sensors, reside in the hand-held MM--a unique and patented aspect of the design. The base unit contains a fluid reservoir, cord reel, stakes, and a 4-ft monopod mount for the MM.

Extracting useful information from pressure noise proved to be one of the biggest challenges. Compulevel samples the sensor output nine times a second, and jolts, jars, pinches, even the swinging of the cord, can create inputs that mask the elevation data. "Early prototypes would never lock on in windy conditions because the criteria is that the computations must be stable to within 0.050 inch," Vories explains. "If you saw the raw sensor output on an oscilloscope, it would be all over the place."

Fuzzy logic incorporated into a microcontroller adjusts for both random and periodic noise inputs. In very hostile conditions, another branch of the fuzzy logic places the unit into a mode that infinitely averages the sensor samples, when the input may not settle to a stable reading.

Fluid selection gave Vories fits as well. It had to be stable, affordable, non-combustible, non-toxic, non-hygroscopic, low in viscosity, and compatible with nearly a dozen containment materials. His top choice met all these requirements, save one: At normal operating conditions, gasses desorbed from the fluid under negative pressure and formed bubbles in the hydraulic system.

All fluids exhibit this property to some degree, yet simply defining the magnitude of the problem proved difficult. "There's very little published information on the absorption of gasses in any liquid other than water," Vories says. After much empirical testing, he ultimately designed the system to be permanently pressurized to prevent gas desorption. The gas bore in the cord communicates this pressure between the fluid-storage reservoir in the base and the reference side of the sensor diaphragm. This approach prevents normal variations in gas pressure from affecting elevation readings.

Compulevel boasts many advantages, not the least of which is the ability to measure relative elevations directly. It only requires one operator and can measure around corners or over obstacles. Compulevel has, in one setup, three times the vertical range of conventional levels, and measures over any elevation on earth without tabulation or manual calculation.

Stanley Tools (New Britain, CT) is manufacturing Compulevel primarily for the construction industry, but other opportunities for the technology exist. It could, for example, be applied to stream or tank fluid-level monitoring, or adapted to backhoes for measuring excavation depth. Explains Vories, "this is a fundamentally new way of measuring things that didn't exist before."

Additional details...Contact Dennis L. Vories, P.E., (619) 749-2502, or for literature (800) 262-2161.

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